Immunological approaches to human prostate cancer depend upon the existence of cell-surface antigens capable of evoking a specific immunological response. The purpose of our study is to determine whether patients with prostatic cancers develop an immunological response to antigens unique for their tumor cell surfaces. In addition to providing information about the fundamental cellular changes underlying malignant transformation, these studies have the immediate practical goals of providing means of assessing the immunological response of patients to their tumors during the course of disease both with and without treatment; of opening avenues for manipulating the immunological response in favor of the tumor-bearing host; and of developing immunological methods for the earlier and more accurate detection of prostatic cancers. Carcinoma of the prostate provides an excellent model for studying cancer immunology in man because of its relatively high prevalence and chronic nature. It is a tumor whose progress can be slowed by surgery and, in some cases, by hormonal therapy or chemotherapy. Because patients with prostatic carcinoma are in relatively good health during much of the course of their disease, they have an opportunity for developing an immune response to the tumors.